I have this small tree that started growing in my garden and I was wondering if I should keep it or remove it; apparently they become very big. If it is a wallnut, what kind is it? I am not a resident of Toronto but not far.

Thank you for contacting the Toronto Master Gardeners. Although it is a bit hard to tell from the photo you sent and, in addition, seedlings can be highly variable, here is some information on the Walnut and Hickory families to try narrowing down the seedling’s ID:
  • Walnuts: members of the family in our area include Black Walnut (Juglans nigra), Butternut (Juglans cinerea) and English Walnut (Juglans regia)
    • Scent: take a leaf and crunch it; if the leaf smells like spiced citrus, or furniture polish, it is most probably a walnut.
    • Leaves: Butternut leaves tend to be rough above and very hairy beneath and sticky when young; Black walnut leaves are smooth above and faintly hairy underneath; English walnut leaves are not serrated.
    • Terminal leaf: English walnuts is large; Black walnut terminal leaves are smaller or even missing at maturity; Butternut about the same size at maturity but can be also large when young.
  • Hickories: Shagbark (Carya ovata) and Bitternut (Carya cordiformis) can be observed in pockets outside the city.
    • Terminal leaf: large; toothed, edges with tuft of hair.

Are there any similar trees on properties near you?  You may have to wait until buds form and woody stems for pith development in order to get a more accurate ID.